This invention relates generally to a method for coupling light from one or more light-generating sources into one or more optical fibers. More specifically, it relates to a method of photolithiographically aligning a laser array to an array of optical fibers (single mode or multimode) which have end faces polished at a 45 degree angle.
Coupling of a semiconductor laser or laser array to an optical fiber or optical fiber array, and, in particular, to single-mode fibers, requires very accurate positioning of the fiber core with respect to the lasing spot of the laser to achieve optimal coupling of the light. The conventional technique typically utilizes a V-groove array which is etched in a substrate such that the V-grooves are photolithographically aligned to the lasers or light sources. The optical fibers are positioned in the V-grooves in a butt-coupling scheme. However, effective coupling of cleaved fibers requires a fiber alignment tolerance, lateral to the beam propagation direction, to within 0.5 .mu.m with respect to the center of the light beam in order to ensure good coupling efficiency. The coupling efficiency can be further improved by tapering the fiber end or lensing the fiber tip. However, these modifications to the fiber further increase the precision necessary for accurate fiber alignment.
The alignment accuracy in the axial direction (along the direction of beam propagation) is less restrictive than in the lateral direction because the coupling efficiency is less sensitive to the precise distance of the fiber to the laser than to the fiber displacement away from the center of the laser beam. In the conventional laser-to-fiber butt-coupling assembly of FIG. 1a, for example, the positioning of the optical fiber in the lateral, y-direction is the least precise but is the most critical since the coupling efficiency is more sensitive to variations in the y and z-directions than in the axial x-direction. For an efficient pigtailing operation, it is desirable to locate the fiber in a fiber receiving conduit, such as a V-groove, to facilitate a permanent fiber attachment in the aligned position. Placing the fiber in such a V-groove, however, allows adjustment of the fiber position in the least critical dimension (x-direction) only, namely along the fiber axis. Additionally, many other parameters of the optical fiber, the laser, and the V-groove substrate affect individual coupling efficiency, or the uniform coupling efficiency of an array.
In co-pending application Ser. No. 07/555,129, filed Jul. 19, 1990 and assigned to the same assignee as the present application, there is shown a method for overcoming some of the disadvantages of the prior art by polishing the ends of the optical fibers at a 45.degree. angle and having the laser beam enter the fiber at a right angle to the fiber axis. While the procedure shown in such application is satisfactory for the purposes therein described, the manual alignment process can be time consuming, particularly if it is desired to couple the light from an array of lasers into corresponding optical fibers.